Electric power systems often drive nonlinear loads which contribute to the generation of harmonics on the power distribution bus of the power system. It is generally desired to keep power quality high on the power distribution bus, and hence, amplitudes of harmonic currents generated by loads are typically regulated or eliminated to relatively small values. In aircraft electric power systems, the impedance of the generator and the distribution bus is relatively high, thus compounding the distortion effects caused by load harmonic currents. It is particularly desirable to keep the magnitudes of low order harmonics at a low level, because these harmonics require large and heavy filters which undesirably add to the size and weight of the power system.
A typical nonlinear load is a three-phase AC/DC rectifier which is used as a front end for various power conversion loads, such as electric motor driven hydraulic pumps, electric motor driven compressors and fans, etc. . . . It is commonly known that the triplen harmonics (i.e., 3rd, 6th, 9th, 12th, . . . multiples of the fundamental) and even harmonics (i.e., 2nd, 4th, 8th, 10th, . . . multiples of the fundamental) are virtually nonexistent in three-phase, no-neutral rectifier applications and accordingly do not affect the system or require filtering. However, the remaining harmonic components for six-pulse rectifiers have amplitudes equal to 1/n where n is the order of the harmonic and is equal to 5, 7, 11, 13, 17, 19, 21, . . . and so on to infinity. The component creating the biggest difficulty is the fifth harmonic, which effectively determines the size and weight of the required filter. Typically, this filter is too heavy and costly to provide a competitive solution, particularly where size and weight must be minimized, as in an aircraft or aerospace environment.
One approach to improving this situation is to increase the number of diodes in the rectifier bridge. For example, by utilizing two six diode bridges (and thus utilizing twelve diodes), the harmonic current distribution changes to n=11, 13, 23, 24, 35, 37, . . . an so on to infinity. Besides the reduction in the quantity of the harmonics, there is a beneficial elimination of the two lowest order (i.e., n=5 and 7) harmonics. As a result, the first harmonic to be filtered (i.e., the 11th) is higher in frequency and less in amplitude than the corresponding harmonic produced by the six diode rectifier bridge (i.e., the 5th). This means the filter requirements will be less for the 12 diode approach than the six diode approach. However, such a rectification circuit requires the use of a phase shifting autotransformer and two current sharing interphase transformers (IPT's), both of which add to the size, weight and cost of the overall circuit.
Further harmonic current magnitude reductions can be obtained by further extending the circuit topology to an 18 or 24 diode bridge; however, even more IPT's and a more complex phase shifting autotransformer must be used. Basically, the penalty for adding diodes to reduce harmonics is always offset by greater size, weight and cost. These factors are particularly detrimental in aircraft and aerospace power system applications.
A still further approach to reducing harmonics is to use what is typically referred to as an "active rectifier." An active rectifier is a standard six diode rectifier supplemented with an active switch (typically a transistor) connected across each diode. A relatively small filter is connected between the active rectifier and the three-phase AC power distribution bus for proper operation of the active filter and for filtering remnant higher order harmonics. By proper control of the active switches, it is possible to draw currents from the power bus with a substantial reduction in the lower order (i.e., harder to filter) harmonic currents. Typically, the active rectifier is controlled in a closed loop fashion to provide transfer switching commands on-the-fly and to provide the desired harmonic control and regulation of the rectified DC voltage at an independently controlled set point. It is commonly accepted in the industry that the basic switching frequency for the transistor(s) must be at least twice the frequency of the highest harmonic to be controlled. For example, in a 400 Hz aircraft power system in which the 5th, 7th, 11th, and 13th harmonics are to be reduced to near zero with an active rectifier, the maximum harmonic to be controlled is the 13th, which is 5200 Hz, and the base switching frequency for the active rectifier must be at least 10.4 kHz. This switching frequency is barely practical for today's IGBT (insulated-gate bipolar transistor) switching devices because at the power levels needed for most aircraft loads, the switching losses at this frequency dominate the total losses. Switching frequencies above 10 kHz create even higher switching losses, and therefore are not generally considered practical with IGBT's. Other potential switching semiconductors such as power FET's and MCT's (MOS-controlled thyristors) are also not practical for various other reasons. Thus, using the active rectifier with this control scheme to control harmonic currents above 5200 Hz is of doubtful practicality.
However, even with these limitations, the active rectifier is a practical harmonic control tool for 400 Hz systems because it can control up to the 13th harmonic, a capability which would require a more complex 18 or 24 diode rectifier and the attendant complex and heavy transformers, or a 12 diode rectifier with a much heaver filter to remove the 11th and 13th harmonics. The phase shifting transformer and IPT's needed for the 12, 18, or 24 diode rectifiers are not needed with the active filter. Thus, on a weight basis, the active rectifier is clearly superior. The active rectifier is also very competitive relative to manufactured costs when it is configured with the new low cost industrial power modules now available on the market which include all the diodes and transistors in a single integrated package. The only drawback for the active rectifier is the control circuit complexity and reliability when compared to the passive 12 diode rectifier, which requires no controls.
A new aircraft electrical power system architecture, called variable frequency, or VF, is currently being considered for aircraft and aerospace electric power systems. The generators used in these power distribution systems operate over a frequency range between 400 Hz and 800 Hz. It is in this application that the active rectifier, as conventionally envisioned, is not competitive due to the switching frequency limitation noted above. Specifically, in order to compete, the active rectifier must be able to control the 13th harmonic at the upper frequency of 800 Hz, i.e., 10.4 kHz. This, in turn, requires a 20.8 kHz minimum switching frequency, which is out of the practical range for power IGBT's and IGBT modules at the needed power ratings of 10 kHz or above.